How to UV-Map for a OpenGL Game
You can pretty much follow any UV unwrapping tutorial on the internet for that. Modeling assets is pretty much the same for all the 3D real-time engines out there. 

However, I would highly suggest, that you learn how to unwrap properly as soon as possible. "Smart UV Project" doesn't necessarily give the best result and as soon as your modeling skills go beyond the primitives (cubes, cylinders and the like) you want to have more advanced texture maps. 

Besides that, I'd highly suggest that you also start with proper material setups and thus with node materials (maybe also with cycles) as soon as possible. You can then bake your materials to texture maps and use them for your game assets. The big advantage here is that you don't need to adjust your UV map to the "direction" your texture takes (e.g. bricks are arranged into a specific direction).

The biggest pitfall when creating 3D game assets is the use of too many materials. Every material needs to be loaded and processed, thus generating CPU and GPU load. Do this many, many times and the FPS of the game will drop. So in the case of a building, it would be advisable to have only one texture map for the walls and the roof. This is also where baking and maybe even the Texture Atlas add-on come in.